Modern heating, ventilating and air conditioning (“HVAC”) systems recycle a large proportion of conditioned air, resulting in improved energy efficiency. Unfortunately, recycling of the air concentrates pathogens in enclosed areas where people congregate, like homes, office buildings and hospitals. This increases the burden on the immune systems of humans who live and work in such enclosed spaces and increases the risk of contracting an airborne infection.
Air handling equipment is also a refuge for microbes. Air ducts are dark and shielded from ambient UV light that inhibits the growth of many types of bacteria in outdoor environments. Condensation that occurs while the system is in cooling operation provides moisture to support growth of microorganisms. Dust particles deposited on the surfaces of ducts and air filters provide nutrients to microorganisms. Such nutrition is especially plentiful on air filters, which process thousands of cubic feet of dust laden air daily. It is now recognized that particulate filters for HVAC systems are propagators of airborne bacteria and are at least partly responsible for transmission of tuberculosis, Legionnaires disease and narcosomal infections in health care facilities. The development of High Efficiency Particulate Air (“HEPA”) air filters has not obviated the problem. HEPA filters are able to trap smaller particles than conventional filters, like some airborne microorganisms, and to hold them but they do not capture all airborne microorganisms and are not equipped to kill the microorganisms that they capture. Effort has been expended to develop HVAC systems with a reduced tendency to propagate microorganisms, with considerable emphasis being placed on the development of effective, long-lasting antimicrobial air filters.
U.S. Pat. No. 3,017,329, which issued in 1962, describes a germicidal and fungicidal filter that is said to decrease the likelihood that objectionable odors or viable germs and spores caused by bacteria or fungi colonies will be thrown off the filter. The filter contains a conventional non-woven filter medium with a coating of germicidal and fungicidal active agent applied either by spraying or bathing. The active agent is selected from organo silver compounds and organo tin compounds, which arc pH neutral and highly toxic to mammals. The active agent can be applied to the non-woven fiber during the conventional manufacturing process of a filter wherein the non-woven fiber is immersed in an aqueous bath containing a binder and optionally a fire retardant. Heating of the treated fabric drives off water, cures the binder and, according to the '329 patent, fixes the germicide onto the filter medium.
U.S. Pat. No. 3,116,969 describes a filter having an alkyl aryl quaternary ammonium chloride antiseptic compound that is held onto the filter fibers by a tacky composition that includes a hygroscopic agent, a thickening agent and a film forming agent.
U.S. Pat. No. 3,820,308 describes a sterilizing air filter having a wet oleaginous coating containing a quaternary ammonium salt as the sterilizing agent.
Dever, M. et al, Tappi Journal 1997, 80(3), 157, reports the results of a study of the antimicrobial efficacy achieved by incorporating an antimicrobial agent into the fibers of melt blown polypropylene air filters. Three unidentified antimicrobial agents were tested individually. Each agent was blended with polypropylene, which was then melt-blown to form the antimicrobial filter medium. Only two of the antimicrobial agents were detectable in the filter medium by FTIR after processing. The blended filter media were tested against common strains of gram positive and gram negative bacteria. Filter media containing the two detectable agents had antimicrobial properties, but the agents also affected the physical properties of the polypropylene by functioning as nucleating agents. Consequently, the polypropylene blends yielded filters with reduced collection efficiencies and thicker fibers than filters made from unblended polypropylene.
Foard, K. K. & Hanley, J. T., ASHRAE Trans. 2001, 107, 156, reports the results of field tests of the antimicrobial efficacy of filters treated with one of three unidentified antimicrobial agents. In field tests where microbial growth was seen on an untreated dust-loaded filter medium, growth also was seen on the treated counterpart. Known antimicrobial filter treatments produced little effect under the conditions in which they arc used.
Kanazawa, A. et al. J. Applied Polymer Sci. 1994, 54, 1305 describes an antimicrobial filter medium prepared by covalently immobilizing antimicrobial phosphonium chloride moieties onto a cellulose substrate. The filter was made by reacting a trialkyl-(3-trimethoxysilylpropyl) phosphonium chloride with the hydroxy groups of the cellulose. The investigators found that the chain length of the alkyl groups on phosphorous affected the potency of the filter but not the packing density. According to their measurements, the density of phosphonium chloride in the resulting filter was in excess of that which would be expected for a monolayer, thus indicating that the phosphonium salts were stacked. More lipophilic phosphonium salts, ie. those with longer alkyl chains, tended to have a higher capacity for capturing bacteria.
Okamoto, M. Proceedings of the Institute of Environmental Sciences and Technology, 1998, 122, discusses the use of silver zeolite as an antimicrobial agent in an air handling filter. According to the investigators, the silver zeolite was attached by a special binder to one side of the filter.
U.S. Patent Publication No. 2001/0045398 describes a process for the preparation of a non-woven porous material having particles immobilized in the interstices thereof by contacting the material with a suspension of particles of predetermined size and urging the suspension through the material so as to entrain the particles in the interstices of the material. The treated material is said to be useful as an antimicrobial barrier.
According to its English language abstract, International Publication No. WO 00/64264 discloses a bactericidal organic polymeric material for filters which is made of a polymer base comprising a backbone and bonded thereto a polymeric pendant group comprising units derived from an N-alkyl-N-vinylalkylamide and triiodide ions fixed to the polymeric material.
International Publication No. WO 02/058812 describes a filter medium containing timed release microcapsules of an antimicrobial agent. The microcapsules contain the agent suspended in a viscous solvent, which slowly diffuses out of the porous shell of the microcapsule. The microcapsules may be held to the fibrous substrate with an adhesive base such as gum arabic.
Other methods of removing infectious airborne microorganisms have been developed. One method uses a device that draws contaminated air into an enclosed chamber where it is percolated through a liquid so that the microorganisms become encapsulated in the liquid. This device suffers from drawbacks. Intimate mixing of the contaminated air with the liquid must be effected in order for the pathogens to be captured and eliminated. This design is not well suited for the high flow rates of a HVAC system and would be awkward and unwieldy to install and service.
Another method uses electrostatic precipitation to disinfect an airstream containing microorganisms, wherein electrostatic precipitation is combined with photocatalytic oxidation as discussed in U.S. Pat. No. 5,993,738. A system of this type uses electricity to charge the particulate matter in the air stream and an opposing grounded collector plate for collecting the charged particulates, wherein a photocatalyst and UV light destroy pathogens accumulating on the collector plates. The most widely available antimicrobial filter system for commercial or residential use, however, employs an ultraviolet light in combination with a filter. For instance, in U.S. Pat. No. 5,523,075 a filter chamber was described as having a series of UV lamps producing a specific wavelength of UV light to destroy airborne bacteria. One drawback of these filtering systems is that it is energy intensive to power the LW lamp and thus very expensive. Commercial HVACs in e.g., hospitals, use this filtration technology as do some home air purifiers (e.g., Ionic Breeze from Sharper Image).
There remains a need for further improvement in anti-microbial air filters. It is one goal of the present invention to provide an anti-pathogenic air filtration medium for air handling systems like HVACs commonly found in commercial and residential enclosed spaces like homes, hospitals, factories, office buildings and the like. Of course, the filter media of the present invention also find use against microorganisms deliberately introduced into the environment by combatants or terrorists. Gas masks typically offer protection against chemical agents, but not against biological pathogens like anthrax, small pox and the like. The filter media of the present invention are able to provide such protection when incorporated into a replaceable filter cartridge of a gas mask. U.S. Pat. No. 6,435,184, which is hereby incorporated by reference, provides a description of a conventional gas mask structure.